For those who don't know SOLID, it's an acronym of acronym.
SOLID stands for the following:
SRP: Single Responsability Principle
OCP: Open/Closed Principle
LSP: Liskov Substitution Principle
ISP: Interface Segregation Principle
DIP: Dependency Inversion Principle
Bringing those principles together is creditted to Robert C. Martin (AKA Uncle Bob).
Please note that I'm also in the process of writing some nice posts on how to apply them to your code.
So stay tuned!